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A coaxial rotor system is a type of rotor system used in helicopters that consists of two rotors mounted on the same axis, with one rotor located above the other. The two rotors rotate in opposite directions, with the upper rotor rotating counter-clockwise and the lower rotor rotating clockwise.

The coaxial rotor system eliminates the need for a tail rotor, which is typically used to counteract the torque produced by the main rotor in a conventional helicopter design. By using two counter-rotating rotors, the coaxial rotor system cancels out the torque, resulting in a more stable and efficient design. So, the torque reaction by the main rotor disc is cancelled out by the rotor system itself.

In a coaxial rotor system, directional control is achieved by changing the pitch angle of the rotor blades on one rotor system independently of the other. This causes the lift generated by one rotor system to be increased or decreased, resulting in the helicopter yawing in the direction of the rotor system with increased lift.